It is well-known that a low-voltage circuit breaker is a switching device having a protection function and has the most basic functions such as overload protection and short-circuit protection, wherein the short-circuit protection is executed through an instantaneous release. As stipulated in UL489 standard, the instantaneous release needs to possess a set function of adjustable release threshold, that is, a release current action preset value (hereinafter referred to as a “release threshold”) of the release can be adjusted by operating a rotary knob on the release, and therefore, the release with such function is generally referred to as an instantaneous adjustable release. The release threshold described here refers to a set value associated with a design allowable maximum of a short-circuit current, and is generally set by a rated current of the circuit breaker. The release threshold adjustable function of the instantaneous adjustable release means that a release current threshold is adjustable, such that one electromagnetic release can satisfy the requirement of adjusting the maximum allowable value of the short-circuit current under different working conditions, or can satisfy the requirements of a circuit breaker in case of different rated currents. A frequently-used instantaneous adjustable release, for example an electromagnetic release, generally comprises an electromagnetic coil, a magnetic yoke, a dynamic iron core, a static iron core and a reset spring. Under normal circumstances, a current flowing through the electromagnetic coil is smaller than the release threshold, the dynamic iron core keeps separated from the static iron core under the action of the elastic force of the reset spring, and an air gap having a certain thickness is formed between the dynamic iron core and the static iron core; when an actual current flowing through the electromagnetic coil is equal to or larger than the release threshold, the dynamic iron core generates a release action immediately, and because the electromagnetic attraction between the dynamic iron core and the static iron core becomes larger than the elastic force of the reset spring, the dynamic iron core can move toward the static iron core against the elastic force of the reset spring till being attracted with the static iron core, and the movement of the dynamic iron core toward the static iron core triggers a trip lever of the circuit breaker to act and renders the circuit breaker to realize instantaneous release trip, thereby playing a short-circuit protection role.
However, The problems of instable action value and poor reliability are generally present in the current instantaneous adjustable release, and especially it is difficult for designers to solve the problems of instable action value and poor reliability for a long time while designing an instantaneous adjustable release of a relatively low current (less than 100 A, for instance). The applicant finds that a function model involved in adjustable release action current has the characteristics of multivariate and complicated function relationships upon the analysis of the reason and is formed by recombining two models, namely an electromagnetic model and a mechanical model. According to the current design of the instantaneous adjustable release, because 1, a variable associated with adjustment of a release action current and a function relationship between the variable and the adjustment of the release action current are ignored, and 2, there is no essential balanced scientific planning among various variant elements of an adjustable release system to result in imbalance of more variant elements owing to simultaneous adjustment and thus result in out of control of physical characteristics and mechanical characteristics of the adjustable release, this must make the adjustable release undergo a series of problems, such as instable action value and poor reliability, and meanwhile the adjustable range of the release threshold cannot be expanded. In a solenoid electromagnetic trip disclosed by the Chinese patent (ZL200820214752.8), the length of a spring is adjusted by a rotary knob to realize linear adjustment of a short-circuit current; a magnetic gap can be trimmed by an adjusting screw assembly arranged on a magnetic yoke to eliminate the inconformity of a release current setting value caused by parts themselves and assembly and solve the conformity of an initial setting value better; however, because this variable of the magnetic field energy of an electromagnetic coil and a function relationship in which this variable is in direct proportion to the quadratic of the release action current are ignored in the prior art, and in addition, the problems of poor instability and poor reliability are still present as it is not considered that a function relationship between the electromagnetic attraction between the dynamic iron core and the static iron core and the release action current is a complicated nonlinear non-trigonometric function relationship, this is a common representative example of an electromagnetic release having an adjustable release threshold designed on the basis of an elastic force balance principle. Therefore, this kind of prior art products certainly has the following defects: 1, the proportion of leakage flux of the magnetic gap in the magnetic field energy of the electromagnetic coil increases significantly as the magnetic field energy of the electromagnetic coil reduces significantly, and therefore, there is a great error between a release current indicated by adjusting the rotary knob and the actual release action current value, and especially in case of a low release current, this error and the instability are more serious, thereby further greatly limiting an adjustable range of the release action current. 2. Because there is no structure of a balance planning designed among variant elements of the electromagnetic release system, the adjusting error of the release current is very to sensitive to a manufacturing error, and even in case of trimming the magnetic gap by an adjusting screw, only the inconformity of initial states of various phases of releases can be improved, without solving the inconformity between the release threshold of the electromagnetic release of each phase under each adjusting state and the actual release action current value. 3. Because the actual release action current value and the elastic force of the reset spring are not in a linear function relationship, the adjustable range of the release threshold is relatively small, and the problems of the working stability and the release reliability under a state of a small release threshold are more prominent. 4. Because it is necessary for the adjustable release based on the elastic force balance principle to arrange the reset spring outside a coil, not only complicated structure, large volume and difficulty to mount and debug will be caused, but also there is also a need to increase such sliding fit pair, such as a short shaft and a sliding groove, so that the parallelism between the reset spring and the movement direction of the dynamic iron core cannot be ensured (it is even impossible to realize coaxiality), and therefore, it is certain to intensify the problems of instable action value and poor reliability of the release action value, as well as large error and instability between the set release threshold and the actual release action current value.